1. Field of the Invention
The present invention relates generally to the fabrication of a dosimeter which is operable to measure the time-weighted concentration of one or more gaseous contaminants in an ambient atmosphere, and more specifically to the selection of particular starting materials, and the fabrication of a diffusive material which permits the passage of a selected gas or gases in a gas mixture solely by a diffusive mechanism in proportion to a concentration gradient of the selected gas or gases across the same diffusive material and independent of the convective movement of the impinging gas mixture. The subject diffusive material adapted to be manufactured in continuous sheets and thereafter formed into any desired sizes or shapes.
2. Description of the Prior Art
The prior art is replete with numerous examples of sampling apparatuses which are operable to detect a wide range of gaseous contaminants in assorted operational environments. For example, in the field of monitoring gaseous contaminants which may reside from time to time in the ambient atmosphere, much of the recent art has been directed towards constructing dosimeters which employ passive means to regulate mass transfer of the particular gaseous contaminant to be detected between the ambient atmosphere containing same and a selected collection medium, thereby eliminating the need for an active pump. Such passive dosimeters typically have operational characteristics which include no moving parts, can be fabricated inexpensively, are simple to utilize, and can be easily affixed to employees for purposes of making personal "breathing zone" measurements. These "breathing zone" measurements permit a more accurate risk assessment to be conducted than what was heretofore possible with the more traditional fixed-site area measurements performed with more elaborate or technologically cumbersome instrumentation.
The authors, Palmes and Gunnison in an article entitled "Personal Monitoring Device for Gaseous Contaminants," American Industrial Hygiene Association Journal, 34, 78-81 (1973) disclosed such a passive dosimeter which measured concentrations of a selected gas by measuring the quantity of the gas which diffused through a single orifice of known size to a collection element, where the concentration of the selected gas was maintained at zero.
Although substantial prior art exists, all passive dosimeters disclosed heretofore have utilized one of only two basic mass transport mechanisms thereby achieving the desired objective of regulated mass transfer. These basic mass transport mechanisms include permeation which takes place through a solid polymeric membrane, and bulk gas diffusion.
Passive dosimeters which employ the mass transport mechanism of permeation typically are configured in a form wherein a solid polymeric membrane is disposed in an attitude between an ambient atmosphere containing a gaseous contaminant to be detected and a collection medium for same. As should be understood, the gaseous contaminant "dissolves" in the solid polymeric membrane and mass uptake occurs when a concentration gradient is established across this same polymeric membrane. The concentration gradient is created, of course, when the collection medium adsorbs or absorbs as appropriate, the contaminant in question. Under steady-state conditions the amount of material which will typically pass into a passive dosimeter in a given time "t" is illustrated by the formula: EQU W=kA(C.sub.a -C.sub.i)t/L (1)
where W=the weight of the material collected expressed in micrograms; k=the empirically determined permeation coefficient expressed in square centimeters per minute; A=the area of the permeation membrane exposed expressed in square centimeters; C.sub.a =the concentration of gaseous contaminant in the ambient atmosphere to be tested expressed in micrograms per cubic centimeter; C.sub.i =the concentration of the gaseous contaminant contained inside the dosimeter (normally zero provided that an efficient collection medium is used); t=the time of sampling expressed in minutes; and L=the length (thickness) of the permeation membrane which is expressed in centimeters. Values of k for polymeric silicon membranes, which are considered the most permeable, and therefore preferred, as disclosed in the reference authored by K. D. Reizner and P. W. West entitled "Collection and Determination of Sulfur Dioxide Incorporating Permeation and West-Gaeke Procedure," Environmental Science and Technology, 7, 526-532 (1973) are indicated as lying in a range between 0.001 and 0.01 square centimeters per minute.
Passive dosimeters which employ the mass transport mechanism of permeation have three major advantages. Firstly mass uptake by the mechanism of permeation which takes place through a solid polymeric membrane appears to be substantially resistant to disturbance occasioned by the variable convective movements of the surrounding ambient atmosphere which are normally encountered during most sampling applications, and therefore no secondary protective design features need be employed. Secondly, the utilization of a solid permeation membrane has the attendant characteristic of being capable of retaining solid or liquid collection media, again without the incorporation of secondary design features. Thirdly, a permeation membrane may be manufactured as a sheet of substantially continuous material therefore allowing considerable latitude with respect to the area and shape of material to be incorporated into a dosimeter.
While the previous prior art devices and practices have achieved numerous laudable benefits, they have a multiplicity of shortcomings which have detracted from their usefulness. For example, dosimeters employing the mass transport mechanisms of permeation have several major drawbacks: (1) The rate of mass uptake, by permeation, must be empirically determined for each selected gaseous contaminant to be detected and frequently for each individual dosimeter because gas "solubilities" cannot in reality accurately be predicted from theory and may vary widely even within single lots of commercially available membrane; (2) very thin and often fragile membranes must be employed to achieve practical sampling rates; (3) the time required to achieve an equilibrium rate of mass uptake can be relatively long as compared with the concentration fluctuations of the gas contaminant to be detected in the ambient atmosphere, hence the contaminant sample collected may not accurately reflect the time-weighted average measure; and (4) the rate of mass uptake may be adversely affected by changes in the temperature or ambient humidity. These several shortcomings have heretofore frequently offset the benefits derived from employing permeation dosimeters, and as a result the preponderance of passive dosimeters now being utilized are of the bulk diffusion type.
It should be understood that the passive dosimeters disclosed to date have generally incorporated one or more substantially still pockets of air which individually have macroscopic dimensions relative to the mean free path length of the gaseous contaminant to be detected, and which is placed between the ambient atmosphere to be sampled, and the selected collection medium. The contaminant is collected according to Fick's First Law of Diffusion which is set forth below: EQU W=pD.sub.b A(C.sub.a -C.sub.i)t/L (2)
where W=the weight of the material collected expressed in micrograms; p=the porosity of the material through which the gas contaminant is diffusing (the fractional void volume, which is generally accepted as being equal to one (1) in an open system); D.sub.b =the bulk gas diffusion coefficient expressed in square centimeters per minute; A=the area of the permeation membrane exposed expressed in square centimeters; C.sub.a =the concentration of the gaseous contaminant contained in the ambient atmosphere expressed in micrograms per cubic centimeter; C.sub.i =the concentration of the gaseous contaminant contained inside the dosimeter (this being normally zero, provided of course, that an efficient collection medium is used); t=the time of sampling expressed in minutes; and L=the length (thickness) of the permeation membrane expressed centimeters. It has been empirically determined that the bulk gas diffusion coefficients for molecules with molecular weights of 300 or less which diffuse through the air lie in a range between 3 and 10 square centimeters per minute under ambient temperature and pressure conditions. It should be understood that molecules with molecular weights of 300 or less are most likely to have a measurable vapor pressure.
Gaseous dosimeters that employ the mass transport mechanism of bulk diffusion to regulate mass uptake of a selected contaminant have several noteworthy advantages. Firstly, dosimeter performance can be accurately predicted from theory by simply substituting diffusion coefficients derived from experiment or theory into equation (2) which was discussed above; Secondly, intra- and inter-dosimeter sampling rates are substantially precise because of the ability to accurately form bulk diffusion channels of substantially uniform dimension; thirdly, the rate of mass transfer is substantially independent of the effects of pressure and humidity and will generally vary with temperature only to an amount expressed as T.sup.1/2 ; and fourthly, the time required to achieve an equilibrium rate of mass uptake typically is rapid as compared to the concentration fluctuations of the gas contaminant in the immediate ambient atmosphere to be tested.
The disadvantages which are attendant with the utilization of bulk diffusion dosimeters are a result of shortcomings inherent in their respective designs. For example, bulk diffusion dosimeters need to incorporate secondary design features to prevent convective air movement, which takes place adjacent to the dosimeter, from disrupting the still pocket of air necessary to regulate mass uptake, and to retain the collection media which might otherwise be released through an open diffusion channel which is generally present in such devices. Two approaches have been utilized to overcome the above noted shortcomings. The first approach has been to employ one or more substantially circular diffusion channels which are individually closed at one end by the collection medium and which further have a ratio of length-to-diameter which exceeds a minimum valve of three. U.S. Pat. No. 4,235,097 and an article authored by W. J. Lautenberger, E. V. Kring, and J. A. Morello entitled "A new personal badge monitor for organic vapors" found in the American Industrial Hygiene Association Journal, 41 737-747 (1980); and the article written by R. H. Brown, J. Charlton, and K. J. Saunders, entitled "The development of an improved diffusive sampler," American Industrial Hygiene Association Journal, 42, 865-869 (1981); discuss this principal in greater detail. In practice, however, ratios of ten or greater are commonly required to satisfactorily attenuate the deleterious effects of convective air movements thereby leading to disadvantageously reduced sampling rates. The second approach is to utilize a sheet of substantially macroporous material as a "wind screen." For example, U.S. Pat. No. 3,950,980 to Brown et al. discloses the use of two or more layers of porous material having effective pore sizes in a range between 0.1 and 100 micrometers which are used to attenuate convective gas movement so as to create one or more thin layers of placid gas within an enclosure. Further, U.S. Pat. No. 3,985,017 to Goldsmith discloses the use of a porous sheet having preferred effective pore sizes in an effective range between 5 and 50 micrometers that permits unhindered diffusion, and which is disposed in an attitude at the entrance of an enclosure containing an internal honeycomb structure that substantially inhibits convective movement. It should be appreciated, however, that the latter problem has been most often addressed by utilizing a self-supporting or otherwise solid collection medium, although parenthetically it should be noted that U.S. Pat. No. 4,265,635 discloses the use of a porous hydrophobic film (50-80% porous having pore sizes in the range of 0.1-3.0 micrometers) placed over the end of a plurality of diffusive channels and operable not to interfere with the passage of gaseous contaminants between the ambient air to be tested and a liquid collection medium.
Therefore, it has long been known that it would be desirable to have a gaseous contaminant dosimeter that can incorporate the performance and design features of both permeation and bulk diffusion type dosimeters while simultaneously avoiding the detriments individually associated therewith, and which further is particularly well suited to being transported easily by employees, can be manufactured in a compact configuration and which is operable to provide accurate concentration measurements for the gaseous contaminants to be detected.